Chemical Kinetics

Chapter 13

Chemical Kinetics

Thermodynamics – does a reaction take place?

Kinetics – how fast does a reaction proceed?

Reaction rate is the change in the concentration of a reactant or a product with time (M/s).

A B

rate = -[A]t

rate = [B]t

[A] = change in concentration of A over time period t

[B] = change in concentration of B over time period t

Because [A] decreases with time, [A] is negative.

13.1

A B

13.1

rate = -[A]t

rate = [B]t

time

Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g)

average rate = -[Br2]t

= -[Br2]final – [Br2]initial

tfinal - tinitial

slope oftangent

slope oftangent slope of

tangent

instantaneous rate = rate for specific instance in time13.1

rate [Br2]

rate = k [Br2]

k = rate[Br2]

13.1

= rate constant

= 3.50 x 10-3 s-1

Factors that Affect Reaction Rate

1. Temperature• Collision Theory: When two chemicals react, their

molecules have to collide with each other with sufficient energy for the reaction to take place.

• Kinetic Theory: Increasing temperature means the molecules move faster.

2. Concentrations of reactants • More reactants mean more collisions if enough energy is

present

3. Catalysts • Speed up reactions by lowering activation energy

4. Surface area of a solid reactant • Bread and Butter theory: more area for reactants to be in

contact

5. Pressure of gaseous reactants or products• Increased number of collisions

The Rate Law

13.2

The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers.

aA + bB cC + dD

Rate = k [A]x[B]y

reaction is xth order in A

reaction is yth order in B

reaction is (x +y)th order overall

F2 (g) + 2ClO2 (g) 2FClO2 (g)

rate = k [F2]x[ClO2]y

Double [F2] with [ClO2] constant

Rate doubles

x = 1

Quadruple [ClO2] with [F2] constant

Rate quadruples

y = 1

rate = k [F2][ClO2]

13.2

Run # Initial [A] ([A]0)

Initial [B] ([B]0)

Initial Rate (v0)

1 1.00 M 1.00 M 1.25 x 10-2 M/s

2 1.00 M 2.00 M 2.5 x 10-2 M/s

3 2.00 M 2.00 M 2.5 x 10-2 M/s

What is the order with respect to A?

What is the order with respect to B?

What is the overall order of the reaction?

0

1

1

[NO(g)] (mol dm-3) [Cl2(g)] (mol dm-3) Initial Rate

(mol dm-3 s-1) 

0.250  0.250  1.43 x 10-6

0.250  0.500  2.86 x 10-6

0.500  0.500  1.14 x 10-5

What is the order with respect to Cl2?

What is the order with respect to NO?

What is the overall order of the reaction?

1

2

3

F2 (g) + 2ClO2 (g) 2FClO2 (g)

rate = k [F2][ClO2]

Rate Laws

• Rate laws are always determined experimentally.

• Reaction order is always defined in terms of reactant (not product) concentrations.

• The order of a reactant is not related to the stoichiometric coefficient of the reactant in the balanced chemical equation.

1

13.2

Determine the rate law and calculate the rate constant for the following reaction from the following data:S2O8

2- (aq) + 3I- (aq) 2SO42- (aq) + I3

- (aq)

Experiment [S2O82-] [I-]

Initial Rate (M/s)

1 0.08 0.034 2.2 x 10-4

2 0.08 0.017 1.1 x 10-4

3 0.16 0.017 2.2 x 10-4

rate = k [S2O82-]x[I-]y

Double [I-], rate doubles (experiment 1 & 2)

y = 1

Double [S2O82-], rate doubles (experiment 2 & 3)

x = 1

k = rate

[S2O82-][I-]

=2.2 x 10-4 M/s

(0.08 M)(0.034 M)= 0.08/M•s

13.2

rate = k [S2O82-][I-]

First-Order Reactions

13.3

rate = -[A]t

rate = k [A]

[A] is the concentration of A at any time t

[A]0 is the concentration of A at time t=0

[A] = [A]0e-kt

ln[A] - ln[A]0 = - kt

Decomposition of N2O5

13.3

The reaction 2A B is first order in A with a rate constant of 2.8 x 10-2 s-1 at 800C. How long will it take for A to decrease from 0.88 M to 0.14 M ?

ln[A] - ln[A]0 = - kt

t =ln[A]0 – ln[A]

k= 66 s

[A]0 = 0.88 M

[A] = 0.14 M

ln[A]0

[A]

k=

ln0.88 M

0.14 M

2.8 x 10-2 s-1=

13.3

[A] = [A]0e-kt

ln[A]0 - ln[A] = kt

First-Order Reactions

13.3

The half-life, t½, is the time required for the concentration of a reactant to decrease to half of its initial concentration.

t½ = t when [A] = [A]0/2

ln[A]0

[A]0/2

k=t½

Ln 2k

=0.693

k=

What is the half-life of N2O5 if it decomposes with a rate constant of 5.7 x 10-4 s-1?

t½Ln 2k

=0.693

5.7 x 10-4 s-1= = 1200 s = 20 minutes

How do you know decomposition is first order?

units of k (s-1)

A product

First-order reaction

# of half-lives [A] = [A]0/n

1

2

3

4

2

4

8

16

13.3

13.3

Second-Order Reactions

13.3

rate = -[A]t

rate = k [A]2 [A] is the concentration of A at any time t

[A]0 is the concentration of A at time t=0

1[A]

-1

[A]0

= ktt½ = t when [A] = [A]0/2

t½ =1

k[A]0

Half life for second order

Zero-Order Reactions

13.3

rate = -[A]t

rate = k [A]0 = k

[A] is the concentration of A at any time t

[A]0 is the concentration of A at time t=0

t½ = t when [A] = [A]0/2

t½ =[A]0

2k

[A] - [A]0 = ktHalf life for zero order

Summary of the Kinetics of Zero-Order, First-Orderand Second-Order Reactions

Order Rate LawConcentration-Time

Equation Half-Life

0

1

2

rate = k

rate = k [A]

rate = k [A]2

ln[A] - ln[A]0 = - kt

1[A]

-1

[A]0

= kt

[A] - [A]0 = - kt

t½

Ln 2

k=

t½ =[A]0

2k

t½ =1

k[A]0

13.3

A + B C + D

Exothermic Reaction Endothermic Reaction

The activation energy (Ea) is the minimum amount of energy required to initiate a chemical reaction.

13.4

Temperature Dependence of the Rate Constant

k = A • exp( -Ea/RT )

Ea is the activation energy (J/mol)

R is the gas constant (8.314 J/K•mol)

T is the absolute temperature

A is the frequency factor

Ln k = --Ea

R1T

+ lnA

(Arrhenius equation)

13.4

13.5

Reaction Mechanisms

The overall progress of a chemical reaction can be represented at the molecular level by a series of simple elementary steps or elementary reactions.

The sequence of elementary steps that leads to product formation is the reaction mechanism.

2NO (g) + O2 (g) 2NO2 (g)

N2O2 is detected during the reaction!

Elementary step: NO + NO N2O2

Elementary step: N2O2 + O2 2NO2

Overall reaction: 2NO + O2 2NO2

+

13.5

Elementary step: NO + NO N2O2

Elementary step: N2O2 + O2 2NO2

Overall reaction: 2NO + O2 2NO2

+

Intermediates are species that appear in a reaction mechanism but not in the overall balanced equation.

An intermediate is always formed in an early elementary step and consumed in a later elementary step.

Reaction Intermediates

Rate Laws and Rate Determining Steps

13.5

Writing plausible reaction mechanisms:

• The sum of the elementary steps must give the overall balanced equation for the reaction.

• The rate-determining step should predict the same rate law that is determined experimentally.

Unimolecular reaction A products rate = k [A]

Bimolecular reaction A + B products rate = k [A][B]

Bimolecular reaction A + A products rate = k [A]2

Rate Laws and Elementary Steps

13.5

A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed.

Ea k

uncatalyzed catalyzed

ratecatalyzed > rateuncatalyzed

13.6

Energy Diagrams

Exothermic Endothermic

(a) Activation energy (Ea) for the forward reaction

(b) Activation energy (Ea) for the reverse reaction

(c) Delta H

50 kJ/mol 300 kJ/mol

150 kJ/mol 100 kJ/mol

-100 kJ/mol +200 kJ/mol

The experimental rate law for the reaction between NO2 and CO to produce NO and CO2 is rate = k[NO2]2. The reaction is believed to occur via two steps:

Step 1: NO2 + NO2 NO + NO3

Step 2: NO3 + CO NO2 + CO2

What is the equation for the overall reaction?

NO2+ CO NO + CO2

What is the intermediate? Catalyst?

NO3

What can you say about the relative rates of steps 1 and 2?

rate = k[NO2]2 is the rate law for step 1 so step 1 must be slower than step 2

13.5

NO2

Write the rate law for this reaction. Rate = k [HBr] [O2]

List all intermediates in this reaction.

List all catalysts in this reaction.

HOOBr, HOBr

None

Ostwald Process

Hot Pt wire over NH3 solutionPt-Rh catalysts used

in Ostwald process

4NH3 (g) + 5O2 (g) 4NO (g) + 6H2O (g)Pt catalyst

2NO (g) + O2 (g) 2NO2 (g)

2NO2 (g) + H2O (l) HNO2 (aq) + HNO3 (aq)

13.6

Catalytic Converters

13.6

CO + Unburned Hydrocarbons + O2 CO2 + H2Ocatalytic

converter

2NO + 2NO2 2N2 + 3O2

catalyticconverter

Enzyme Catalysis

13.6